Several methods are known for sealing optical semiconductor devices such as, e.g., light-emitting diodes, in resin. A casting molding method consists of pouring a liquid thermosetting or UV-curable resin into a mold, inserting a lead frame carrying a light-emitting diode element connected by means of bonding wires to the frame, and then curing the resin by heating or UV radiation. An injection molding method consists of melting a thermoplastic resin in the cylinder of an injection molding machine, injecting the resin under pressure into a cavity of a mold in which a lead frame carrying a light-emitting diode element connected by means of bonding wires to the frame was inserted, and then curing the resin. According to another method that is known as a transfer molding process, pressure is applied to liquefy a B-stage resin which is stopped in the middle of the curing reaction, a lead frame carrying a light-emitting diode element connected by means of bonding wires to the frame is inserted in a mold, and the resin was supplied into the mold, and then the resin is cured by heating (see Japanese Laid-Open Patent Application Publication (Kokai) (hereinafter referred to as “Kokai”) Sho 54-19660, Kokai Sho 57-2583, Japanese Examined Patent Application Publication Hei 4-40870, Kokai Hei 9-107128, and Kokai 2003-8082).
At the present time, sealing of light-emitting diodes is normally carried out with the use of a thermosetting resin, such as an epoxy resin. However, an increase in intensity of light emitted by such optical semiconductor device and operation of the device in a continuous mode change the color of the cured body of the sealing resin and decrease reliability of the device. Normally, sealing of light-emitting diodes is carried out by casting molding. However, the casting molding method is associated with a number of problems such as low production efficiency caused by slow curing of thermosetting resins, need for the use of several jigs for positioning of the resin sealing unit and lead frame that may lead to formation of defects in the obtained light-emitting diodes, possibility of development of dry spots on the surfaces caused by shrinkage at curing of thermosetting resins, formation of voids in the sealing resin, and separation of the sealing resin from the light-emitting diode element that may be caused by concentration of curing stress on the light-emitting diode element.
The transfer molding method is advantageous in that this method allows accurate control of thickness in the sealing resin layer. Disadvantages of the transfer molding method consist of the following: variations of pressure in the sealing resin flow may deform, break or create undesired contact of bonding wires that are used for electrical connection of the light-emitting diode; the method may require the use of cooling means needed to stop curing of aforementioned thermosetting B-stage resin during storage and transportation thereof; and extra cost associated with the factors mentioned above.
On the other hand, advantages of injection molding method reside in the fact that a light emitting diode element is placed directly into the mold, the lead frame connected to the light emitting diode element is fixed directly in the mold, a smaller number of molds may be needed for production, the obtained light emitting diodes have smaller deviations in properties, the products are to a lesser degree subject to formation of defects, and do not develop dry spots caused by shrinkage at curing. Nevertheless, injection molding may cause breakage and undesirable contact of bonding wires that are used for electrical connection of light emitting diodes elements.
It is an object of the present invention to provide a method of sealing optical semiconductor devices, such as, e.g., light-emitting diodes, that does not form voids in the sealing material, allows accurate control of thickness in a sealing resin layer, does not cause breakage or undesired contacts of bonding wires, is characterized by low concentration of stress in optical semiconductor device elements, reduces chance of discoloration of the sealing resin and disconnection of the sealing resin from the optical semiconductor device elements, even after long use or after operation at increased intensity of light generation, and is suitable for industrial production of optical semiconductor devices of high reliability. It is another object to provide a sealed optical semiconductor device that even after a long use possesses excellent reliability, and is not subject to discoloration of the sealing resin layer and to decrease in the brightness of the generated light that may be caused by peeling of the sealing resin layer from the light-emitting semiconductor element.